| Calliandra calothyrsus - Production and use: A Field Manual |
James M. Roshetko, Didier Lesueur, and JeanMichel Sarrailh
Calliandra calothyrsus is adapted to a wide range of environments. When planting this species, however, it is important to use a provenance (seed source) that is known to perform well in an environment similar to that of the planting site. Additionally, the selected provenance should provide needed services or products such as control of soil erosion, overstory shade, and fuelwood, fodder, or seed production. Identifying the best provenance for a site will usually result in increased tree survival, growth, and productivity.
Seed selection. Seed of C. calothyrsus is more delicate than seed of most other leguminous tree species. Some seedlots may germinate well during testing, yet produce weak seedlings that perform poorly in the nursery or field. To ensure high quality, only collect seed that is fully mature. Only purchase seed from reliable sources. At a minimum, sources should provide documentation on where the seed was collected, the number of trees the seed was collected from, and the date of collection. To maintain seed quality, always store seed under optimum conditions—in sealed containers in a refrigerator at 4°C.
Seed treatment. Simple seed treatment can overcome the protective function of the seed coat and encourage good germination. Cutting or scraping a small hole through the seed coat, called "nicking," is recommended for small quantities of seed. A knife, nail-cutter, or file can be used for this procedure. To avoid damaging the seed embryo, cut the seed coat opposite the micropyle or hilum. After nicking, soak seed in cool water for 12 to 24 hours before sowing. The nicking treatment is not practical for large quantities of seed. Instead, soak seed in cool water for a minimum of 24 hours. With either treatment, seed should be sown immediately.
Soaking seed in hot water for 2 to 5 minutes and then in cool water for 12 to 24 hours usually produces good results. However, soaking in very hot water may damage the seed. Test the hot-water treatment on small samples for different lengths of time before treating large quantities of seed. Mechanical scarification machines (similar to grist mills or threshers) have been used to scarify large quantities of seed quickly, but this method is inconsistent and sometimes damages the seed.
Inoculation with Rhizobium. Like many other legumes, C. calothyrsus can form a symbiotic relationship with the soil bacterium, Rhizobium spp. The trees provide the rhizobia with carbohydrates for energy, and the rhizobia convert atmospheric nitrogen from the soil into a form that the trees can use. This process, called nitrogen fixation, occurs in nodules that develop on the roots of the trees.
There are many strains of Rhizobium in different soils around the world, but only some strains form a symbiotic relationship with C. calothyrsus that leads to effective nitrogen fixation. It is easy to determine whether the rhizobial strains that occur naturally in your soil are fixing nitrogen. Simply check the roots of C. calothyrsus trees for nodulation. Nodules should be abundant, and when you cut them open they should be red or pink inside. If root nodules are green, brown, or black, they are not fixing nitrogen.
In areas where C. calothyrsus is native or has been naturalized for some time, the soil may well contain rhizobia of the appropriate strains. If C. calothyrsus is not common to the area, however, or if the site is degraded, there may not be enough of the correct rhizobia in the soil to stimulate nitrogen fixation. In such situations, it is best to treat seed or seedlings with a selected strain of rhizobia to stimulate maximum nitrogen fixation. This process is called inoculation.
For C. calothyrsus, the Nitrogen Fixation by Tropical Agricultural Legumes (NifTAL) Center in Hawaii recommends a mixture of rhizobial strains that are also compatible with Gliricidia septum and Leucaena leucocephala. If rhizobial strains are present in the soil, however, they may compete with introduced strains. Where indigenous rhizobia are present, trials should be conducted to compare tree growth in the presence of indigenous and introduced strains. The best-performing strains can then be chosen for inoculation.
The material used to inoculate seed or seedlings is produced by isolating rhizobia from the root nodules of healthy trees, growing them in a laboratory, and mixing them with peat. The peat-based inoculant, which is manufactured and distributed by several government agencies and private commercial firms, contains 1,000 times the number of rhizobia normally found in the soil. Because rhizobial inoculants are inexpensive and the potential benefits are great, inoculation is recommended under most circumstances.
Rhizobium inoculants are available from the suppliers listed in Appendix A. When ordering inoculants, you should specify the C. calothyrsus provenance you are planting and the environmental characteristics of the planting site, such as average annual rainfall, maximum and minimum temperature, elevation, soil type, and soil pH. This information will help the supplier select the best available strain.
When using or storing a rhizobial inoculant, remember that the bacteria it contains are alive. They are sensitive to heat, drying out, freezing, and exposure to direct sunlight. If not used immediately, the inoculant should be tightly sealed and stored in a moist, cool, dark place. It should not be frozen. Follow the application and storage instructions provided by the supplier.
The rhizobial inoculant should be applied directly to C. calothyrsus seeds immediately before planting. Just after soaking the seeds, cover them with a sticker solution to make sure that the inoculant adheres to them. The NifTAL Center (Keyser 1990) recommends placing seeds in a plastic bag or bucket and covering them with a solution of gum arabic, sugar, or vegetable oil. Either dissolve 40 g of gum arable in 100 ml of hot water and allow it to cool, or dissolve 1 part sugar in 9 parts water. Combine 2 ml of one of these mixtures, or 2 ml of vegetable oil, with 100 g of seeds, and shake or stir the mixture vigorously until the seeds are evenly coated. Then add 5 mg of inoculant and shake or stir again until the seeds are well coated. Allow the inoculated seeds to dry for 10 minutes to eliminate any stickiness, and sow immediately. Do not store inoculated seeds—the rhizobia will die.
Seedlings can also be inoculated in the nursery. Mix inoculant in cool water, and irrigate the seedlings with the suspension. Keep the mixture well stirred or shaken, and irrigate until all the inoculant is washed into the root zone. One 50 g bag of inoculant is enough to inoculate 10,000 seedlings.
Alternatively, seedlings can be inoculated with alginate beads. These are produced in a laboratory and contain pure cultures of select Rhizobium strains. Before application, the beads must be rehydrated for 10 hours in a 0.1 M phosphate buffer of pH 7.4. Five grams of beads require I liter of buffer. If phosphate solution is not available, rehydrate 5 g of beads for 24 hours in I liter of water. Apply the solution to seedlings at a rate of 16 g per nursery bag.
Diem and others (1989) provide detailed information on the production of alginate beads. Somasegaran and Hoben (1985) provide information on the production of peat-based inoculants. General information on nitrogen fixation is available in Postgate (1987) and Roskoski (1989).
Inoculation with mycorrhizae. Calliandra calothyrsus also forms an important symbiotic relationship with vesiculararbuscular mycorrhizae (VAM). These are immobile fungi that live in the organic layer of the soil. By growing in filaments out from a tree's roots, the mycorrhizae increase the tree's root area and thus improve access to soil moisture and nutrients. This relationship is especially important in arid environments and in soils low in nutrients such as acidic, phosphorous-deficient soils. The best way to insure that C. calothyrsus benefits from the VAM symbiosis is to inoculate trees with the appropriate mycorrhizae.
The most common method of VAM inoculation is to take soil from the organic layer beneath a healthy C. calothyrsus tree and mix it with nursery soil at a rate of 5 to 10 percent by volume. This method is practical for most small-scale farm or community nurseries. The main disadvantage is that soil pathogens may be introduced into the nursery. Treating the soil with chemicals or hot water to kill pathogens is not recommended because the treatment will also kill the mycorrhizae.
Establishing a VAM "production bed" in the tree nursery is another method of inoculation. The first step is to collect soil from under a healthy C. calothyrsus tree. Place the soil in a nursery bed and sow Calliandra seeds at a spacing of 5 x 5 cm. The seedling roots and associated mycorrhizae will grow throughout the production bed. Once this occurs, dig up the soil along with the roots, chop the mixture finely, and combine it with nursery soil at a rate of 5 to 10 percent by volume. This method is more expensive and labor intensive than the first, and it has the same disadvantage of possibly introducing soil pathogens into the nursery. Nevertheless, if a large number of seedlings are to be grown for several consecutive years, the construction and management of a VAM production bed may a worthwhile investment.
To avoid the risk of introducing soil pathogens, you may prefer to order a VAM inoculant from one of the suppliers listed in Appendix A. As with rhizobial inoculants, you should specify the C. calothyrsus provenance you are planting and the environmental characteristics of the planting site. Although little information is available on the specificity between C. calothyrsus and particular strains of VAM fungi, commercial inoculants are now available that are claimed to be effective with most tropical legumes.
When using or storing a VAM inoculant, remember that it contains live fungi, which are sensitive to heat, drying out, freezing, and exposure to direct sunlight. If not used immediately, the inoculant should be tightly sealed and stored in a moist, cool, dark place. It should not be frozen. Follow the application and storage instructions provided by the supplier. More information on VAM inoculation is available in Castellano and Molina (1989), Ferguson and Woodhead (1982), and Malajczuk and others (undated).
Nursery Production. In the nursery, treated and inoculated seeds should be sown directly into plastic bags. These may vary in size from 5 x 15 cm to 15 x 25 cm when pressed flat. They should always contain drainage holes at their base. The choice of bag is determined by the desired seedling size: if large seedlings are desired, use large bags. Large seedlings compete well in the field and need less care after planting than small seedlings. Small seedlings, on the other hand, are easier and less expensive to grow and to transport.
Fill nursery bags with a fertile nursery mixture several days before seeds are treated and inoculated. A good-quality nursery mixture is three parts soil, one part sand, and one part compost. These components should be thoroughly mixed. The compost should be completely decomposed—uncomposted manure may spread harmful pathogens. The soil mixture should have a pH of 5.5 to 7.5. Fill bags with the mixture, and pack gently to close all air pockets. To promote settling, irrigate the soil until water drains from the holes at the base of the bag. Inoculate the soil with a VAM inoculant.
Sow seeds that have been treated to stimulate germination and inoculated with rhizobia to a depth equal to their width. The planting hole can be filled with the nursery mixture or clean sand. Water bags daily. Germination will occur in 4 to 10 days. When seeds are of good quality and you expect high germination, sow one seed per bag. When you anticipate low germination, sow two seeds per bag. If both seeds germinate, transplant one to an empty bag. Weak or malformed germinants should be discarded.
Depending on local environmental conditions, you should water seedlings once or twice a day. Water should penetrate to the bottom of the bags and drain freely. The soil surface should be dry before watering again. Provide germinants and young seedlings with 50 percent shade during their first month or half the period of their stay in the nursery. After this, gradually reduce shade to harden-off seedlings before transplanting. In most areas, seedlings will be ready for transplanting after 6 to 12 weeks in the nursery. Seedling height may vary from 15 to 50 cm. Evans (1982), Jackson (1989), and Liegel and Venator ( 1987) provide more information on nursery-management techniques.
Direct Sowing. Calliandra calothyrsus may be established by sowing seeds directly into the planting site. However, seedlings produced in this way are more susceptible to climatic extremes than are seedlings grown in a nursery. For this reason, seeds used for direct sowing must be of the highest quality. They should be treated and inoculated as in the nursery and planted to a depth equal to their width. The spacing between seeds will be determined by the management objective. In most areas, seeds are sown by hand. However, in Australia and New Caledonia where labor costs are high, C. calothyrsus seeds are commonly sown by a mechanical "band seeder." If using such equipment, do not soak seeds during pretreatment because swollen seeds will jam the machine.
Young germinants and seedlings are slow growing and vulnerable to competition from other plants. For this reason, it is important to prepare the site thoroughly before sowing. You should kill or remove all competing vegetation, including roots, within 40 to 50 cm of sowing positions. This can be accomplished by manual, mechanical, or chemical means. Continue controlling competing vegetation until trees are well established. You can also improve tree establishment and growth by loosening the soil at the planting site. Soil cultivation is particularly beneficial on heavy soils with poor water infiltration.
Stump sprouts. Calliandra calothyrsus can be successfully propagated by stump sprouts made from seedlings or wildlings. Make cuttings from seedlings that are 4 to 12 months old. Stumps made from older seedlings often show poor growth and vigor. Only healthy, straight seedlings should be selected. They may be as tall as I meter with root collar diameters of 1 to 2 cm. Remove seedlings from the nursery or forest soil. Cut their stems at 10 to 30 cm above the root collar, and remove all remaining foliage. Cut their taproots at 10 to 20 cm below the root collar. Survival is greatest when stumps are planted immediately, but they may be stored for up to one week in a place that is cool, dry, and shady. Stumps should be planted so their root collar is flush with the soil surface. Because stumps are vulnerable to desiccation, they should be planted at the beginning of the rainy season.
Vegetative propagation. Calliandra calothyrsus can be propagated vegetatively from young, succulent seedlings or root sprouts. Prepare an air-tight propagation box, called a "polypropagator," by lining the bottom and sides with a polyethylene sheet, and adding a bottom layer of gravel, a second layer of fine gravel, and a third layer of sand, topped with a uniform mixture of sand and sawdust. Each layer should be 3 to 5 cm deep. An airtight poly-propagator loses very little moisture, but you will need to water the planting medium to maintain a relative humidity of 80 percent.
Harvest stems from young seedlings or root sprouts early in the morning, and transfer them immediately to the propagation area. Succulent stems are very sensitive to desiccation—delays of more than I to 2 hours will result in high mortality. Cut stems into small pieces, 5 to 7 cm long, each containing two to three leaflets. Place cuttings in the poly-propagator at a spacing of 5 x 5 cm to 10 x 10 cm. Keep them in the poly-propagator for one month, and mistspray them every 2 to 3 days with water. Then transplant cuttings into plastic nursery bags, provide them with shade for one week, and gradually expose them to full sunlight. After transplanting cuttings into bags, you should maintained them for at least two months in the nursery before planting them in the field. Longman (1993) describes this propagation method in detail.
Although simple and useful, this method adds a month to seedling production schedules, requires additional work, and is not widely known. In most areas, its use is not warranted. However, once superior provenances and land-races of C. calothyrsus are identified, this propagation method may be a suitable for treeimprovement projects. It may also be useful in areas where C. calothyrsus seed production is limited.
Transplant seedlings at the beginning of the rainy season into pits prepared two to four weeks ahead of time. Dig pits at least 25 cm wide and 25 cm deep, cultivate the soil finely, and return it to the pit. Just before planting, slit each plastic nursery bag down one side, and remove the seedling carefully without disturbing the ball of soil around its roots. If roots are encircling the soil ball, cut them on two sides of the ball with a sharp, clean knife. This will keep the roots from growing in a ball after transplanting.
Place each seedling in the middle of a planting pit. The top of the soil ball should be level with the soil surface. Gently pack the loose soil around the seedling so it is perfectly straight. If soil moisture evaporation is a problem, cover the soil surface around the seedling with mulch. Be sure, however, that the mulch does not contain weed seeds.
Regardless of how they are established, all young C. calothyrsus seedlings demonstrate slow initial growth. During this period, they are very vulnerable to competition from other plants for sunlight, moisture, and soil nutrients. Fast-growing pasture grasses, such as Bothriochloa petusa (silver grass), Bracharia mutica (pare grass), and Panicum maximum (guinea grass), have dense root systems in the upper soil layers and are particularly competitive with young seedlings. Such competition must be controlled if C. calothyrsus seedlings are to grow well. In general, all vegetation within 40 to 50 cm of the seedlings should be removed every one to three months. Weed control may be necessary for 6 to 12 months or until the seedlings dominate competing vegetation.
In arid conditions, however, neighboring vegetation may actually protect C. calothyrsus seedlings from sun and wind. Lowgrowing ground cover may also protect the soil surface from desiccation. Under such conditions, removing neighboring vegetation may increase seedling mortality and decrease growth. This situation has been observed on shallow, infertile soils in Jamaica.
Studies have shown that phosphorus fertilizers can improve C. calothyrsus growth and promote nitrogen fixation with symbiotic Rhizobium. Although precise fertilization regimes have not yet been determined, in New Caledonia the standard recommendation is to apply 100 kg of 32-16 phosphoruspotassium fertilizer per hectare.